Comment by sowbug

Malice isn't enough. The only special power of a 50% miner is to consistently resolve double-spend attempts in its favor. If this happened, it would surely lower the value of Bitcoin. This, an attacker must be not just malicious but also irrational enough to forfeit the $800,000 in Bitcoin that it creates daily by virtue of its 50% control of the network. (6 blocks/hour x 24 hours/day x 25 bitcoin/block x $450/bitcoin x 50%)